Conventional reel unit projection systems use horizontal platters to supply film to a projector and take-up the film that comes out of the projector in such a way that film roll rewinding is not necessary. Controls and configuration of conventional film transport systems that have been used for decades are described in patents by Willi Burth (U.S. Pat. No. 3,780,959), and Edwin M Potts (U.S. Pat. No. 3,823,890). These types of film transport systems supply film to the projector by removing film from the center of the film roll via a pay-out device located concentrically on the supply platter. Film that has gone through the projector gets wound onto a removable hub that is located concentrically on the take-up platter. At the end of the show the hub on the take-up platter is removed so that the film can be removed via the pay-out device at the center of the platter for the next showing. The removed hub is inserted onto an empty platter to take up film again. With the conventional type no-rewind film transport systems, there is a drive motor for each platter. The supply platter controller uses a control feedback signal derived from the pay-out device film position sensor or sensors to maintain or help maintain the correct angular supply platter speed. The take-up platter controller uses a signal from a dancer arm to maintain the correct angular take-up platter speed. The dancer arm typically actuates an analog device such as a potentiometer.
These film supply and take-up transports are designed to be as low a cost as possible and to minimize time required of the projectionist for film handling. U.S. Pat. No. 4,169,566 by Angelo Boudouris et al. is another variation of the same type of film transport, but with a simplified drive system with the intent to further reduce the cost of the film transport unit. The only projectionist involvement required for no-rewind film transports is the short time it takes to rethread the film transport and the projector prior to each show. These systems have a history of being effective and play an important part in keeping the theatre capital and operating costs down.
When these conventional film transports are scaled up to handle larger film loads they become more expensive and suffer a performance limitation for a wide range of film platter loads. The supply platter control system can not be tuned to one set of parameters that will achieve stable operation for film roll sizes that range from a few pounds (i.e. trailers) to several hundred pounds. When the roll size is such that the tuned parameters are not able to control the supply platter smoothly, side effects such as film roll distortion, film roll shifting, film roll cinching, and supply platter control instabilities can cause the film to become damaged. Cinching is when a section of the film roll begins to move with respect to another section of the film roll. Repairing of large format film prints, such as 15/70 format, or repairing equipment damaged by the high forces that occur when Estar based film is pulled apart is expensive and generally considered unacceptable by theatre operators.
The limitation of these conventional systems lies in the ability of the supply platter control system to control a wide range of film platter loads. In conventional no-rewind film transport system the supply platter feedback mechanism are all similar in that they leave a pay-out device at the center of the supply platter. The pay-out device acts as a mechanical interface that allows film to be drawn off the inner radius of the film roll and redirects it to the film roller network that transports film to the projector. Position of the film leading into the pay-out device is fed back to the supply platter controller to control the supply platter. The section of film leading into the pay-out device can be referred to as the “lead-in” film. These pay-out devices on conventional no-rewind film transports generally all have the similar limiting range of detecting lead-in film position. When the film lead-in position exceeds the detecting limit the lead-in film begins to wrap its self about the outer circumference of the pay-out device. It is this narrow feedback range used to control the wide range of film platter loads that limits the supply platter control system's ability to remain stable.
Supply platter instability usually occurs during and just after the supply platter ramps up to running speed that leads to the side effects mentioned above. During ramp up the supply platter should pay out film at the rate that matches the rate the projector requires film. If there is a small rate mismatch the lead-in film position remains within the limits of the film position sensing range. As the rate mismatch increases the position of the lead-in film moves outside the position sensing limits. When film begins to wrap it self about the pay-out device there is no additional feedback to indicate the magnitude of the lead-in film positional error. To correct for the rate mismatch the supply platter has to be accelerated, if the platter is moving too slowly, or decelerated if it was moving too quickly. The platter angular speed continues to change to get the film back to within the film position sensor's sensing range. When within the sensing range the supply platter acceleration or deceleration condition is removed. The longer the supply platter has been accelerating or decelerating the greater the chance the lead-in film will quickly pass through the position sensing range. When this happens the film wraps itself about the pay-out device in the opposite direction at which time the supply controller will decelerate the platter that is moving too quickly or accelerate the platter if it is moving too slowly. This over and undershoot cycling will continue, sometimes for quite a while, until the lead-in film remains settled within the positional sensing range. Conventional configurations of the way film leads into the pay-out device generally do not provide a way to increase the film position sensing range by any dramatic amount.
A factor that contributes to the difficulty of keeping the lead-in film within the film position sensing range is being able to determine a value of platter acceleration or deceleration used for platter angular speed correction that will work for both the smallest and largest film platter load at any one specific inner roll radius. The torque required to accelerate a film platter load is proportional to the rotational inertia of the film and platter. Given that the rotational inertia of a film roll is proportional to the film roll radius to the fourth power, then it becomes readily apparent that the rotational inertia for a few pounds of film will be hugely less than 500 lbs. of film. There is also the difference in rotational inertia between the small and large platters that will contribute further to the problem.
Generally there are no feedback parameters available on conventional film transport systems to indicate platter load size to the platter controller and, thus, it is not possible to modify the torque used to accelerate the platter accordingly. If a constant torque is used to accelerate the film platter load then the magnitude of the ratio of the angular acceleration between small and large platter load will be proportional to the magnitude of the rotational inertia ratio. Since the difference in rotational inertia between small and large platter loads is large this will mean that ratio of acceleration between small and large platter loads will be large for a given applied torque. A problem occurs in that the required torque profile needed to accelerate a large film platter load for stable platter control will be excessive for small film platter loads leading to control instabilities. In these situations the under and over-shoot cycling continues to diverge from reaching the steady state operating condition and the projectionist has to stop the projector. If the projector is not stopped, such as when the projectionist is not present, too much film wraps around the pay-out device and tightens onto itself causing extensive damage to the print and possible damage to the projection equipment. In other situations the oscillations continue over a long enough period of time, the motor heats up substantially causing a thermal cutout condition to occur. Both scenarios lead to a situation of a lost show or shows.
Generally, conventional supply film position sensing systems that have been used extensively to date have similar lead-in position sensing range and do not take film platter load size into account in the platter control system. The following patents describe different film supply position sensing systems used in no rewind film transport systems: a swing arm with optical cam analog feedback is disclosed in U.S. Pat. No. 3,823,890; a swing arm with feedback from limit switches is disclosed in U.S. Pat. No. 4,169,566; and optically sensing film position is disclosed in U.S. Pat. No. 5,992,780.